i want to know what difference between SDR ram and newly comes DDR ram technologies. which is the best?

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SDRAM stands for Synchronous Dynamic Random Access Memory. DDR is short for "DDR SDRAM" and stands for Double Data Rate. Nowadays to avoid misunderstandings SDRAM is often specified as SDR SDRAM and SDR stands for Single Data Rate by analogy with DDR. Therefore, the main difference between SDR and DDR memory the doubled speed: DDR can transfer data at roughly twice the speed of SDRAM. For example, PC133 SDRAM runs at 133 Mhz, while 133 Mhz DDR effectively runs at 133 Mhz x 2 = 266 Mhz.
SDRAM has 168 pins and two notches at the connector, which prevents it from being used in a DDR SDRAM motherboard and vice versa. It comes mainly in PC66, PC100 and PC133; the bus speeds of the RAM in MHz.
DDR SDRAM has 184 pins and a single notch at the connector. It comes in speeds of PC1600 (166 MHz), PC1800 (200 MHz), PC2100 (266 MHz), PC2700 (333 MHz), PC3200 (400 MHz), and PC4400 (550 Mhz). The numbers represent the theoretical maximum bandwidth of the DDR SDRAM in Megabytes per second (MB/s). For example, PC2100 has a theoretical maximum bandwidth of 2100 MB/s.